In order to satisfy present-day and future environmental demands in relation to the production of chemical pulp, novel methods have been developed or are under development for the final delignification and bleaching of pulp.
For several decades, chlorine and chlorine compounds have been utilized in the bleaching of cellulose, but an ever increasing environmental awareness and a trend towards closing the pulp production process to an ever increasing extent, thereby rendering more difficult the return of residual products from the bleaching to the recovery system, has had the result that bleaching chemicals which do not contain chlorine are now being used to an ever greater extent. In recent years, an evermore widespread use of an alkaline oxygen bleaching stage following the delignification in the digester vessel has markedly decreased the need for bleaching chemicals for the final bleaching. In spite of this, the bleaching chemicals are still responsible for a considerable part of the costs of producing bleached pulp.
An alternative to chlorine-containing bleaching chemicals is to use different forms of peroxide compounds, such as, for example, hydrogen peroxide, which have been used in industrial bleaching since the beginning of this century. However, bleaching pulp solely with peroxides is usually insufficient to meet the demands placed by the market on brightness, inter alia. The use of peroxides in combination with other chlorine-free bleaching chemicals, such as, for example, ozone, is rapidly gaining ground and provides a good pulp quality with regard to both brightness and strength. The use of peroxides, including both inorganic peroxides, such as hydrogen peroxide and sodium peroxide, and organic peroxides, such as peracetic acid, has been tested out with favorable results in connection with bleaching pulp. However, hydrogen peroxide is the compound which is most frequently employed and it has several advantages from the environmental point of view as well as good commercial availability.
However, the cost of the hydrogen peroxide, which is responsible for the major part of the total cost of bleaching to a given brightness, represents an important disadvantage. As a consequence, peroxides, despite their advantages, have hitherto only been utilized to a small extent for bleaching pulp and then usually in the final stages of the bleaching process.
The main object of the present invention is to improve the prerequisites for using the environmentally advantageous hydrogen peroxide in connection with bleaching paper pulp. It has been found that, with the aid of the invention, it is possible, in a very advantageous and somewhat surprising manner, to produce peroxides within the pupling digester works starting from readily available raw material, mainly by the gasification or partial combustion of cellulose spent liquors in order to generate the hydrogen gas which is necessary for manufacturing the hydrogen peroxide.
The method which is nowadays by far the most prevalent for producing hydrogen peroxide is the so-called AO process, or anthraquinone process. In the AO process, an alkylanthraquinone is hydrogenated in the presence of a catalyst to give the corresponding hydroquinone, which in turn is oxidized by oxygen or air with the formation of hydrogen peroxide. The hydrogen peroxide is extracted with water and the quinone which is reformed is returned to the hydrogenation stage, thereby completing the loop. The process has been carried out commercially for several years and is now well established. Solutions of hydrogen peroxide in water are commercially available in concentrations up to 90 per cent by weight, but 35-70% strength solutions are most common in connection with bleaching. The starting materials for hydrogen peroxide production are hydrogen and oxygen, or air. Oxygen is presently used to an ever increasing extent within the cellulose industry, particularly for delignifying pulp, and is therefore available at most factories. Hydrogen is not normally present and is not currently used within the cellulose industry. Industrial production of hydrogen mainly occurs within the petrochemical industry and the alkali metal chloride industry, the main areas of use for hydrogen being the manufacture of ammonia and methanol.
The hydrogen is normally produced by the gasification of different hydrocarbons, such as, for example, tar, liquid petroleum gas or hard coal. A disadvantage of producing hydrogen gas from these raw materials is that the carbon dioxide (CO.sub.2) which is produced at the same time does not originate from a biomass fuel. Emission of carbon dioxide originating from the gasification or combustion of non-biomass fuels is considered to constitute a less desirable contribution in the atmosphere and is therefore subject to a tax or charge in many countries. In principle, all raw materials containing hydrogen can be used for producing hydrogen gas by gasification, and since 1988 a gasification plant in Finland has been in operation for producing hydrogen gas from peat.
In principle, all gasification processes can be utilized for producing hydrogen gas. The highest yield is obtained by gasifying with oxygen at high temperature, resulting in a synthesis gas containing in the main hydrogen and carbon monoxide. The carbon monoxide can be reacted (shifted) with water, with the formation of hydrogen and carbon dioxide, in accordance with the water gas reaction. The reaction is carried out in one or more shift reactors coupled in series. In the delignification of wood according to the sulphate cellulose method, a water-containing residual product is obtained which also contains an organic fraction consisting in the main of lignin compounds, oxidized carbohydrates and organic extracted matter, and an inorganic fraction containing alkali metal salts. Normally, the residual product or the black liquor, which is therefore a biomass fuel, is burnt to recover energy and chemicals according to well-known and established technology.
However, it has emerged that partial combustion or gasification of the black liquor can provide important advantages. A technique for partial combustion or gasification which is particularly suitable when applying the present invention is the so-called CHEMREC technique, which, inter alia, is described in U.S. Pat. Nos. 4,601,786, 4,808,264 and SE-466 268.
However, other gasification techniques, such as, for example, gasification in a fluid bed, can also be used when applying the present invention.
Gasification of black liquor is already practiced today on a commercial scale and is expected to increase in importance. Other spent liquors and lignin-containing materials occurring within the cellulose industry, for example bleaching plant effluent concentrates, can also be gasified in suitable equipment for recovering both energy and chemicals, or alternatively used for producing hydrogen gas in accordance with the present invention.